The invention relates to a measuring sensor for noninvasive detection of electro-physiological quantities.
In medical and physiological measurements, electrodes are used to detect a multiplicity of electrical signals (for example electrocardiogram, electromyogram, electroneurogram, electroencephelogram, etc.) for purposes of diagnosis and therapy. Thus, for example, French Patent Specification No. 2,088,780 describes an EEG electrtode with a plurality of individual needle-like pins which are fastened to a common baseplate at a short distance from one another and which, during measurement, do not penetrate into the skin, but improve skin contact and transition resistance by means of a locally higher surface pressure. A further example of electrodes of this type, to which reference may be made, is the conducting electrode described in German Offenlegungsschrift No. 3,025,955 for detecting bioelectrical activity of hairy parts of the body. Such reference discloses a plurality of conducting pins mounted in an electrode housing so as to be displaceable counter to compression and contact springs which ensure sufficient contact pressure against the skin surface. The signals to be derived are generated inside the body which can be conceived as a volume conductor with an ionconducting medium of moderate specific conductivity. In electrical terms, the skin represents an enveloping surface of the volume conductor and has at the same time a very high complex specific resistance. The electrical and mechanical properties of the skin therefore play a special part, since it can influence the measurement result greatly (see G. Rau "The Influence of Electrode and Skin Impedance in Measurements with Surface Electrodes (EMG)", Biomedizinische Technik 18, 23 to 27 (1973)).
The use of invasive electrodes which are pricked through the skin into the body, for example in the form of wire or needle electrodes, largely eliminates the influences of the properties of the skin on the measurement. However, other disadvantages have to be taken into account at the same time. The examinations are painful, which also presents serious problems especially when children are examined, and are scarcely tolerable when the measurements are repeated. There is also an increased potential danger of infection, since the skin, as a physiological barrier, is penetrated completely. Consequently, possibilities of non-invasive measurement are sought, and if at all possible, surface electrodes are preferred.
However, the use of surface electrodes again presents a series of problems indicated below merely by key phrases:
(1) The transfer from the electrode material to the electrolyte;
(2) The electrical and mechanical coupling of the skin;
(3) The electrical and mechanical properties of the skin itself.
The skin has a very high complex electrical resistance, for example impedance values of several M.OMEGA./cm.sup.2 can be observed with measuring currents of a frequency of 1 Hz. This resistance is also highly non-homogeneous over the extent of the skin surface. Most of the electrical impedance is centered in the horny layer, that is to say in the stratum corneum conjunctum, namely up to approximately the tenth to fourteenth cell layer from outside. The outermost cell layers are in the process of dying and being cast off (see P. Schulz, "New Methods and Investigations of the Electrical Resistance of Human Skin Under Direct and Alternating Currents", thesis Freiburg 1966).
Also, when pressure is exerted on the skin surface, (a) a variation or modulation of the skin resistance occurs, and (b) a voltage is generated on the skin between the inside and ouside (skin potential). In any measurement, these effects produce interfering signals which are called "movement artefacts". These also include the disturbances caused by relative movements of the electrode surface and the skin surface; these are reduced by means of different methods of fastening the electrodes, such as adhesion, clamping with rubber bands, suction and the like. A customary method by which an attempt is made to eliminate the skin influences by scraping off or cutting off the uppermost skin layers often causes considerable injury together with a danger of infection.
Another possibility of reducing the influence of movement artefacts on the measurement, but without having to scrape off or cut off the upper cell layers of the skin, is mentioned in German Offenlegungsschrift No. 2,555,281 which is the starting point for the invention. It is known from this publication to fashion points on measuring-sensor electrodes, so that they penetrate such a little way into the skin surface that only the uppermost horny cell layers are pierced. However, these are large-area electrodes having a plurality of points which each form a unit in elctrical terms.